The present invention relates generally to radiation detectors and, more specifically, to enhanced sensitivity backside illuminated radiation detectors particularly adaptable for the detection of long wave infrared radiation (LWIR).
Naturally, in the design and construction of high quality radiation detectors, the desire is to make the detector as sensitive as possible to incident radiation within a desired range of frequencies. A generally well known cause of limited sensitivity is a generic phenomenon known as dark current. This phenomenon encompasses a number of different mechanisms operating simultaneously, though perhaps independently, within the radiation detector. These mechanisms, however, are similar in that they result in the flow of current through the detector irrespective of whether there is incident radiation. Thus, the flow of current in the absence of meaningful illumination gives rise to the generic term dark current.
Detector sensitivity is lost in direct proportion to the amount of dark current that flows through the detector. Since dark current effectively generates noise in proportion to its current density, significant dark current flow directly results in the reduction of the detector's signal to noise ratio. Thus, the substantial, if not complete, inhibition of any of the constituent dark current mechanisms will yield a distinct improvement in the sensitivity of the radiation detector.
As an example, a generally known dark current mechanism is thermal charge carrier generation. In the case of a donor impurity type radiation detector, electrons are ionized from their associated impurity atoms by the absorption of thermal energy. These ionized electrons move from the impurity level to the conduction band of the crystal lattice. They are then swept by an electric field to the positive radiation detector electrical contact. The electric field, of course, is created by a voltage potential difference applied across the radiation detector. Additional electrons are injected from the negative potential electrical contact. The net effect, therefore, is a dark current flowing through the radiation detector independently of any current induced by incident radiation.
A method of substantially inhibiting dark current due to the thermal generation mechanism is equally well known. Since thermal energy is required for the mechanism to operate, reducing the temperature of the radiation detector to within a few degrees of absolute zero effectively freezes out the mechanism. Accordingly, the percentage of impurity band electrons in the conduction band due to radiation absorption ionization is increased, resulting in a greater detector sensitivity to incident radiation.
Another known dark current mechanism is gamma radiation induced charge carrier generation. Radiation detectors are naturally designed and constructed so as to be as insensitive as possible to incident radiation of all such frequencies that fall outside their particularly desired frequency detection range. However, some percentage of incident radiation of any given frequency will be absorbed by a practical radiation detector. Due to the high energy of charge carriers generated by gamma radiation absorbtion, additional charge carriers are subsequently generated through electron collisions. This charge carrier multiplication results in a substantial dark current. Consequently, gamma generated dark current is of particular concern in the case of radiation detectors intended for operation in environments subject to significant amounts of gamma radiation.
This particular sensitivity to gamma radiation is heightened in the case of most conventional radiation detectors. Typically, they utilize high volume, low doping density radiation detection regions for the absorbtion of incident radiation. The low doping density provides for a low conductivity detection region as needed to inhibit the ordinary flow of current from the applied bias voltage potential through the impurity band of the detection region. The high volume of the detection region compensates for the low doping density as necessary to maintain an acceptable radiation absorbtion efficiency. This, however, increases the sensitivity of the detector to gamma radiation. The high volume of the detection region affords gamma radiation a greater statistical opportunity to be absorbed. Consequently, most conventional radiation detectors operate inaccurately, if at all, in the presence of significant quantities of gamma radiation.
As mentioned above, there are a wide variety of mechanisms that result in the generation of dark current. Some of these mechanisms are fairly well understood and methods of inhibiting their operation have been devised. Others, including the impurity band conduction mechanism, are less well understood, if at all.
It is also desirable, with regard to the design and construction of high quality radiation detectors, that they be particularly adaptable to a wide variety of applications. These applications may range from the simple detection of a given radiation wavelength to the high resolution imaging of complex radiation sources. Thus, the radiation detector must be adaptable for use as a discrete device as well as in high density focal plane arrays (FPA). Further, with regard to its use in FPA's, the radiation detector must be compatible with a wide variety of read-out structures, including hybridized thin film circuitry and monolithic charge coupled device (CCD) circuitry. The use of a hybrid readout structure in conjunction with an FPA generally requires that the radiation detector be capable of operation in a reverse or backside illuminated mode.